Application of MgCl 2 ·6H 2 O for thermochemical seasonal solar heat storage

IRES 2010 November 22-24, 2010, Berlin, Germany Topic area: Thermal Energy Storage Preferred presentation form: oral Application of MgCl2.6H2O for thermochemical seasonal solar heat storage H.A. Zondag, V.M. van Essen, L.P.J. Bleijendaal, B.W.J. Kikkert and M. Bakker ECN, Energy Research Centre of the Netherlands, P.O. Box 1, 1755 ZG Petten, The Netherlands, telephone: +31 224 56 4941, fax: +31 224 56 8966, email: zondag@ecn.nl 2 Eindhoven University of Technology, Department of Mechanical Engineering, P.O.Box 513, 5600 MB Eindhoven, The Netherlands Introduction The heat demand in the summer can be completely fulfilled using solar heat, but in the winter the heat demand is exceeding the solar supply. A solution is to store the excess of solar energy in the summer, and to use it to fulfill the heat demand in the winter. Water is traditionally used for storing heat (e.g. solar boiler), but seasonal heat storage requires large water tanks (>40m) that are too large to be placed inside a family building. An alternative option is to store heat by means of chemical processes using the reversible reaction: A+B⇔C+heat. With thermochemical heat storage, energy storage densities can be reached that are ten times higher than for heat storage in water. Additionally, after the thermochemical material has been charged, the heat can be stored for a very long time without losses. Interesting materials are cheap, non-toxic, non-corrosive, have sufficient energy storage density and have reaction temperatures in the proper range. A large materials inventory by ECN identified a number of interesting materials, including magnesium chloride hexahydrate (MgCl2.6H2O) as one of the most promising materials for seasonal heat storage (Zondag, 2007). Material characterization In order to verify the performance of the magnesium chloride hydrate, the sample mass was measured as a function of temperature using TGA equipment, as shown in Figure 1. When heating the material at a rate of 1°C/min, these measurements showed that the dehydration of the hexahydrate started at about 50°C and the formation of monohydrate started at about 120°C. These results indicate that the dehydration of the material occurs in a temperature range that is within reach of solar thermal collectors. From the molar masses of the magnesiumchloride hexahydrate and the the desorbed water molecules, the dehydration of hexahydrate to monohydrate can be calculated to reduce the sample mass to 55.7%, which is in reasonable agreement with the 57.2% found in the measurement. Figure 1: TGA measurement of MgCl2x6H2O. Heating rate 1°C /min in dry nitrogen atmosphere, sample mass 10 mg. Next, hydration measurements were carried out in an evacuated setup, as shown in Figure 2. During hydration with moisture from an evaporator at 25°C (corresponding to a vapor pressure of 32 mbar), the temperature in the sorption bed rose from 25°C to 85°C. With an evaporator temperature of 10 C (vapor pressure 12 mbar) and an initial reactor temperature of 50°C, the temperature in the sorption bed rose from 50°C to 70°C. 0 10 20 30 40 50 60 70 0 50